Reportedly, previtamin D.sub.3 may be obtained from 7-dehydrocholesterol (7-DHC) by irradiation. This previtamin may be converted by thermal rearrangement into vitamin D.sub.3, which is thermally more stable.
Various sources of irradiation have been considered to drive the 7-DHC to previtamin D.sub.3 reaction. For example, the process of forming photons from excimer or exciplex reactions is known from laser technology. Laser photon sources, however, are not suitable for photochemical synthesis of previtamin D.sub.3 because of their high technical complexity and the fact that their radiation geometry has little suitability for preparative photochemistry and the associated radiation density is insufficient over a large area.
Conventional photochemical synthesis of previtamin D.sub.3 on an industrial scale reportedly has been effected by irradiation of 7-DHC using medium-pressure mercury lamps. Because the starting material (7-DHC), the primary product (previtamin D.sub.3) as well as byproducts, absorb with different efficiency in the same wavelength range, polychromatic radiation of the kind supplied by these lamps favours the formation of photochemical byproducts which are inactive and in some cases toxic. Therefore, with the present state of the art, it is necessary to interrupt the irradiation after relatively low conversion of the 7-DHC to previtamin to D.sub.3. The unconverted 7-DHC is recycled while the primary product (previtamin D.sub.3) must be purified in an expensive working up procedure.
Filter effects are a further consequence of substrates and products which absorb in the same wavelength range. For example, when the absorption spectrum of previtamin D.sub.3 overlaps completely with that of 7-DHC, the previtamin absorbs a continuously increasing proportion of the light as the conversion proceeds.
Another reason for interrupting the conventional reaction after a relatively low conversion (10-20%) of 7-DHC to previtamin D.sub.3 is the fact that the quantum yield (i.e., the efficiency) of the subsequent photochemical reaction of previtamin D to, e.g. tachysterol, is greater than the quantum yield of the formation of the desired product (previtamin D.sub.3). Thus, in conventional reactions, the efficiency of the reaction is decreased while the cost of production of the end product is increased.
Another significant problem during conventional production of previtamin D.sub.3 is the poor correlation between the emission spectrum of medium-pressure mercury lamps and the absorption spectrum of 7-DHC. Thus, in conventional processes using medium-pressure mercury lamps only about 1% of the radiation radiating therefrom is in the desired range, i.e., between about 280 and about 300 nm. Moreover, because the radiation spectrum produced by a conventional medium-pressure mercury lamp is not optimized for the 280-300 nm wavelength, a large amount of undesired byproducts are produced by irradiation outside this optimum wavelength region.
Accordingly, one object of present the invention is to provide a photolytic process for the production of previtamin D.sub.3 from 7-dehydrocholesterol, which process has fewer disadvantages than the prior-art procedure.
The present invention is directed to meeting this and other objects.